1. Field of the Invention
The invention relates to power amplifiers, and in particular to gain control of power amplifiers with low noise figure.
2. Description of the Related Art
Power amplifiers are widely employed in Radio Frequency (RF) transmitters or receivers in applications requiring the transmission of electromagnetic energy including GSM (Global System for Mobile Communications), EGSM (Extended GSM), PCS (Personal Communications Service), DCS (Digital Cellular System), 3G cellular applications such as EDGE (Enhanced Data Rates for GSM Evolution), CDMA (Code Division Multiple Access), WCDMA (Wideband CDMA), WiFi, UWB (Ultra Wideband), microwave communications, and the like.
In the case of an RF receiver, the received signal has a high dynamic range. Since baseband operates at a fixed voltage, a power amplifier with gain controllability is required in the RF receiver to convert wide dynamic range of the received signal to a constant. In transmitters, a power amplifier receives constant input signal levels and provides a varying range of output signal levels.
A noise figure (NF) is a ratio of actual output power to expected output power without noise introduced by the power amplifier, and is a function of amplifier gain and internal noise injection. To achieve a low NF, low noise amplification or higher power amplifier gain is employed. It is desirable to have a low NF in a power amplifier such that minimal transmission power may be achieved. However the high power amplifier gain for low NF compromises low power consumption for battery life. Thus there is a need for a power amplifier capable of gain adjustment at lower power consumption, to provide a better control over both.